1. Field Of The Invention
The present invention relates generally to processes and systems for separating oil, water and gas, and more particularly, but not by way of limitation, to oil well production separation systems.
2. Description Of The Prior Art
The fluid stream produced from a typical oil well is a three-phase stream containing oil, water and gas. To a lesser extent, the stream may also include solids or other contaminating fluids which are not relevant to the present invention. Conventional production fluid separation systems deliver this three-phase production stream into a three-phase gravity separator tank. A gas stream is withdrawn from the upper portion of the gravity separator tank. Oil and some water flows over a weir and is withdrawn into a conduit often referred to as an oil leg of the separator. Water with some contaminating oil remaining is withdrawn from a conduit often referred to as a water leg of the separator. One or more additional three-phase gravity separators may be placed in series with the first gravity separator with the oil leg of the first separator being directed into the second separator, and so on. The water legs of each of the gravity separators may be subjected to additional treatment to remove additional oil from the oily water discharged therefrom. De-oiling hydrocyclones are often used for further treatment of the oily water from the water leg of the gravity separators. A float tank may be used downstream of the de-oiling hydrocyclones for final degassing and further de-oiling of the water before it is discharged back to the sea. FIG. 1 of the present application depicts a typical prior art separation system of the kind just described which is currently commonly used on offshore production platforms in the North Sea. The de-oiling cyclones utilized in systems like that illustrated in FIG. 1 are designed for removal of relatively small concentrations of oil from water, and may be constructed in accordance with U.S. Pat. No. 4,576,724 to Colman et al., and U.S. Pat. No. 4,764,287 to Colman et al.
The prior art also includes hydrocyclone separators particularly adapted for removing relatively small concentrations of water from an oil stream. Such hydrocyclones are generally referred to as dewatering hydrocyclones and may for example be constructed in accordance with the teachings of U.S. Pat. No. 4,749,490 to Smyth et al.
In both de-oiling hydrocyclones and dewatering hydrocyclones, the emphasis is on the quality of one of the streams exiting the hydrocyclone.
In the case of the de-oiling hydrocyclones, the emphasis is on the quality of the water or underflow stream, and it is generally acceptable that the overflow stream will contain both oil and water. With the de-oiling hydrocyclone, the general purpose is for the water underflow stream to be sufficiently pure that it can be returned to the sea or perhaps reinjected into the formation.
With the dewatering hydrocyclones, on the other hand, the emphasis is on the quality of the overflow oil stream.